Light-emitting devices (LEDs) are commonly used in a back-lighting source for a liquid crystal display (LCD) panel. In particular, LEDs in red, green and blue colors are used to provide a back-lighting source in “white” color. In prior art, when a driving circuit is used to drive a display having one or more strings of light-emitting devices (LEDs), these strings are connected in parallel to form a single current supply path. As shown in FIG. 1, a current limiting device and a current limiting resistor Rcl are used to regulate the total current in the current supply path. In such a driving circuit, a voltage boosting device is used as a power supply to supply the current to the LEDs. Alternatively, a current sensing device is used to provide a feedback to the voltage boosting device in order to regulate the total current in the current supply path, as shown in FIG. 2.
In the driving circuits as shown in FIGS. 1 and 2, it is assumed that the current through each of the string of LEDs is substantially the same. However, because the non-linear relationship between the voltage drop and the current in an LED, one or more slightly irregular LEDs in a string may cause the current through that LED string to increase significantly. As such, the useful operational life of the LEDs in that string may be significantly shortened. If the strings of LEDs are used to provide in a white back-lighting source, the color balance in the back-lighting source may be shifted because the brightness in one string is different from the brightness in other strings.
It is possible to use a separate driving circuit for each string of LEDs. For example, a current regulator with a voltage upgrade feature can be used to regulate the current through the LED string. As shown in FIG. 3, the current regulator regulates the current by sensing the voltage across the current sensing resistor Rcs. While this type of current regulator is very effective in regulating current, it is not a cost-effective solution. Furthermore, this type of current regulator produces a significant amount of electromagnetic radiation that could be a problematic source of electromagnetic interference (EMI).
Alternatively, a group of LEDs of the same color can be connected in parallel and each parallel current path has a separate current limiting resistor in a voltage regulator as shown in FIG. 4 and in a current regulator as shown in FIG. 5. However, the electrical characteristic of the LEDs in each parallel current path must be examined and matched so that the currents through the parallel current paths can be equalized.
It is thus desirable and advantageous to provide a method and a device that is cost effective and effective in regulating the current in each group of color LEDs in a back-lighting source.